1. Field of the Invention
The present invention relates to a recordable optical recording medium of LTH (Low to High) recording type, having a recording layer that contains organic dye and capable of recording and playing back data using light of 300 nm to 500 nm in wavelength. To be specific, the present invention relates to a recordable optical recording medium of LTH (Low to High) recording type, having a recording layer that contains organic dye and being capable of recording and playing back data using light of 300 nm to 500 nm in wavelength, wherein said recordable optical recording medium permits a light transmission layer of one-layer configuration and whose characteristics do not deteriorate when recording is conducted in a high-temperature environment.
2. Description of the Related Art
LTH-type BD-R discs with a conventional recording layer that uses organic dye material have a cover layer of two-layer structure (the number of layers excludes a hardcoat layer). One of these layers is a cover layer made of hard material to resist scratching, as with HTL-type BD-R discs whose recording layer uses inorganic material; while the other layer is a cover layer that uses soft (low in elastic modulus (20 MPa or less)) material (acrylic resin or pressure-sensitive adhesive) for the purpose of accepting deformation caused by heat generation and expansion of dye when recording is conducted, maintaining a deformed state, and ensuring sufficient characteristics with Δnd. To realize low-cost organic BD-Rs, however, it is desirable to have a cover layer of one-layer configuration made of hard material, as with inorganic BD-Rs. An azo metal complex having a specific molecular structure is proposed as an organic dye material for recordable optical recording media having such cover layer of one-layer configuration (Japanese Patent Laid-open No. 2010-33639 and Japanese Patent Laid-open No. 2010-143184). One feature of this dye is that it is subject to less deformation when recording is conducted and consequently achieves a cover layer of one-layer configuration. However, a slight deformation still occurs during recording even when this dye is used. Since the cover layer is hard, deformation that occurs during recording is not local deformation of the recording pit, but it covers a wide range including the land and even adjacent pits. When recording is conducted in a high-temperature environment, the elastic modulus of the cover layer drops and therefore the amount of deformation increases, which causes a layer separation to occur easily. As explained above, it is difficult for the aforementioned recordable optical recording medium of Blu-ray Disc type, whose recording layer is formed using an azo metal complex of specific molecular structure and which has a light transmission layer (cover layer) of one-layer configuration, to fully absorb and accept deformation of the recording layer and adjacent layer when data is recorded in a high-temperature environment. This has been the cause of deteriorating recording and playback characteristics as the recording layer and adjacent layer separate partially due to the residual stress generated when the recording pit is formed.